In recent memory, the air transportation industry has grown to become the primary mover for priority freight, mail and people. This massive task, and the ever increasing cost of aircraft have combined to focus management attention on efficient aircraft usage and scheduling maintenance to maximize utilization. Thus, aircraft maintenance is done, insofar as possible, as ground service on the ramp during routine flight stops. Such measures characterize the cost management efforts needed to be competitive in the industry.
One aspect of these maintenance procedures is checking landing gear tire and strut pressures and hydraulic accumulator charge pressures. When a pressure reading is below the prescribed minimum, it is restored to the proper level using a high pressure nitrogen cylinder. The cylinders, containing gaseous nitrogen at pressures of over 3,000 p.s.i., are stored in an indoor supply area for safety when not in use, and are taken out onto the ramp only if needed.
Nitrogen cylinders are heavy, weighing upwards of 150 pounds each, and are used in pairs, one being equipped with a pressure reducing regulator for output pressures of from 1,200-3,000 p.s.i., as might be required by hydraulic accumulators, landing gear struts, and the other with a regulator for pressures of from 0-400 p.s.i., the range normally required for aircraft tires. When the pressure in a cylinder falls below 3,000 p.s.i., it is too low for charging struts but more than adequate for servicing tires or operating pneumatic jacks, hence the preference for using them in pairs.
The preferred servicing procedure is to start at the nose gear on one side of the airplane and work back along that side of the fuselage to the main gear struts and tires. After finishing the main gear, the service person crosses under the rear portion of the fuselage, as far forward of the tail as possible, to do the main gear and any other work required on the opposite side. This sequence avoids having to go around the nose end, past the tow tractor and through an area of relatively high activity.
The usual conveyance for cylinders is a conventional welder's cart, which is designed to carry a set of oxygen and acetylene cylinders side by side, between a pair of large diameter, outboard wheels. When the welder's cart is stationary, the cylinders rest on a floor contacting bottom plate in a vertical position. When the cart is to be moved, it is tilted back, so as to lift the bottom plate from the floor and shift the weight of the cart onto the wheels. The welding cart width, across the outboard wheels is usually about 32", which is acceptable for passing through a typical 36" doorway, even if the door is only opened 90.degree.. Welding carts however, are not intended for use on a concrete ramp because the hard steel wheels do not roll freely over expansion joints and other obstacles. Rather, if the steel wheel hits an expansion joint, or any obstacle, it either bounces high enough to clear the obstacle, or slams to a stop so that the cylinders try to tip forward. Once out to the airplane, the height of the nitrogen cylinders creates another potentially serious problem. The cylinders are 60" tall by themselves and, with regulator assemblies installed, are approximately 65" tall. The mechanic can tip the welding cart back to decrease its overall height but, if tipped more than about 20.degree., the cart becomes quite heavy and hard to manage. Commercial aircraft in general have less than 65" ground clearance under the fuselage or engine pods and the mechanic must be very careful in moving and positioning the cylinders to service tires and struts. The steel wheels of the welding cart can make such careful positioning difficult, increasing the likelihood of denting the aluminum aircraft skin. When a welder's cart is positioned in any given location it is placed in an upright position, with the nitrogen cylinders in a vertical attitude. While this is safe under normal circumstances, the cylinders are only nine inches in diameter and five feet tall, so that stability can not be assured in an otherwise minor accident. The worst case scenario would be letting a cylinder fall or hit something so as to break off a pressure reducing regulator. This would vent pressurized nitrogen to the atmosphere and turn the cylinder into a spinning, bouncing, battering ram. For the foregoing reasons, handling a welding cart on the ramp is considered risky, so that two people are generally assigned to the task as a safety precaution.
The alternative is to block the cylinders in place on a rubber tired, flat bed material handling cart, which is awkward, but doable, and forget about going through 36" wide doors. This would mean using the large roll-up hanger doors at the expense of wasted time and heating/cooling losses. All in all, the ease of loading the cylinders on a welder's cart makes it the least objectionable of the two alternatives.
Therefore, the first object of the present invention is to provide an improved apparatus for handling and conveying pressurized nitrogen cylinders in the proximity of aircraft by minimizing the potential for incidental contact with aircraft. A second object is that this improved apparatus must be easily and safely loaded with pressurized nitrogen cylinders. Another object is to provide protection for the pressure regulator assemblies so as to mitigate the possibility of unintended breakage and a fourth object is to provide this improved apparatus in a form that is readily moved through standard width doors.